As a treatment method of varicose veins, a method in which a catheter is inserted into a vein and the vein is occluded by irradiating a wall of the vein with laser light through the catheter is known. In Japanese Patent No. 3406335, a laser light irradiation device for medical use in which a transparent sphere is welded to a distal end of an optical fiber and a reflecting film is formed on the surface of the transparent sphere is described. An irradiation window is made in the reflecting film. Light that goes out from the distal end of the optical fiber and enters the inside of the transparent sphere is dispersedly reflected evenly (i.e. reflected a large number of times) by the inner surface of the reflecting film and is radiated to the external through the irradiation window. That is, the transparent sphere with the surface on which the reflecting film is provided functions as an integrator.
In the laser light irradiation device for medical use described in Japanese Patent No. 3406335, the light that has entered the transparent sphere through the optical fiber remains inside the transparent sphere while being reflected a large number of times by the inner surface of the reflecting film. Therefore, the attenuation of the light until radiation of the light to the external through the irradiation window is large. Furthermore, the energy of the light is transduced to heat in this attenuation and the transparent sphere or the optical fiber can be damaged by this heat.